1. Field of the Invention
The present invention relates to a plastic optical element, an optical scanning device employing the plastic optical element, and an image forming apparatus employing the optical scanning device.
2. Discussion of the Background
There are optical scanning devices for use in multi-color image forming apparatuses that form images on image bearing members according to image information. Such optical scanning devices guide each of multiple laser beams of light emitted from multiple laser light sources onto the image bearing member via deflectors and image formation devices.
Digital photocopiers or laser printers which form color images by simultaneously irradiating four photoreceptor drums arranged in the transfer direction of output sheets with beams of light corresponding to respective photoreceptor drums to form latent images thereon, developing the latent images with different colors of yellow, magenta, cyan, and black with development devices, and overlapping the developed images while sequentially transferring them to the output sheets, have been introduced in recent years into the market to satisfy demand for multi-color image forming apparatuses that produce quality images at a high speed.
In such image forming apparatuses, multiple scanning devices are used for optical scanning, which requires a large space for arrangement of the multiple scanning devices, resulting in a size increase of the image forming apparatus. To deal with this issue, for example, unexamined published Japanese patent application publication no. (hereinafter referred to as JP-A) H04-127115-A describes an arrangement in which multiple beams of light pass through image formation lenses arranged in a stack, i.e., laminate manner, via a single deflector before scanning.
Furthermore, JP-H10-148777-A describes an approach in which image formation devices are provided for each beam of light entering into a single deflector and focus images on corresponding image bearing members while scanning. The image formation device has an integrated structure of multiple optical elements laminated along the sub-scanning direction, i.e., latitudinal direction. Due to this laminated structure, the space required for the intervals between deflectors can be reduced, or the structure itself can be constituted as a single deflector. This lessens the burden on a motor that rotates the deflectors and makes size reduction possible.
With regard to the image formation elements, i.e., optical elements, installed, their cost is reduced by changing the material from glass to plastic and by mass production. In addition, the number of functional elements (parts) is also reduced due to introduction of non-spherical forms, such as longitudinally asymmetrical forms.
With regard to the optical scanning device having a single deflector and image formation elements arranged facing the deflector and laminated in the latitudinal direction, any lapse in quality among the image formation devices shows up as a variability, i.e., a relative positional shift, between individual colors of yellow, magenta, cyan and black.
To be specific, an error in the dimensional accuracy of the image formation elements, i.e., variability in the curvature error component, and variability in a high frequency error component obtained after subtracting the curvature error component at each image height, results in variability (relative positional shift) in optical performance of individual colors such as beam spot diameter, and scanning position.
Variability, i.e., relative positional shift, of the scanning position in the sub-scanning direction, accompanied by variability of the high frequency error component at each image height in the sub-scanning direction among each image formation element, is a fatal problem for a color image forming apparatus.
In this connection it is to be noted that when the image formation element is made of plastic, the dimensional accuracy of the lens phase depends on (1) processing accuracy of mirror-finish inserts, and (2) molding processing.
At the same time, when a typical mirror-finish insert is used, inaccuracy in mounting caused by laminating the image formation elements leads to variability in the scanning position, i.e., relative positional shift, along the sub-scanning direction, resulting in the fatal problem of color shift described above.
The adverse impact on dimensional accuracy and optical performance such as beam spot diameter, and scanning position relating to fixing accuracy is particularly pronounced with continued improvement in the quality of images produced by current image forming apparatus.